renturn 错误

 package ui.cfg;

 import java.awt.BorderLayout;
 import java.awt.FlowLayout;

 import javax.swing.JButton;
 import javax.swing.JFrame;
 import javax.swing.JPanel;
 import javax.swing.JTabbedPane;

 import util.FrameUtil;

 public class FrameConfig extends JFrame{

     private JButton btnOk = new JButton("确定");

     private JButton btnContrl = new JButton("取消");

     private JButton btnUser = new JButton("应用");

     public FrameConfig() {
         //设置布局管理器为“边界布局”
         this.setLayout(new BorderLayout());
         //添加主面板
         this.add(createMainPanel(),BorderLayout.CENTER);
         //添加按钮面板
         this.add(this.createButtonPanel(),BorderLayout.SOUTH);
         this.setSize(512, 256);
         //居中
         FrameUtil.setFrameCenter(this);
         //TODO测试用
         this.setDefaultCloseOperation(3);
         this.setVisible(true);
     }

     /**
      * 创建按钮面板
      */
     private JPanel createButtonPanel() {
         //创建按钮面板，流式布局
         JPanel jp = new JPanel(new FlowLayout(FlowLayout.RIGHT));
         jp.add(this.btnOk);
         jp.add(this.btnContrl);
         jp.add(this.btnUser);
         return jp;
     }

     /**
      * 创建主面板
      * @return
      */
     private JTabbedPane createMainPanel() {
         return null;
     }

     public static void main(String[] args) {
         new FrameConfig();
     }
     }

源码如上，下面是报错：

Exception in thread "main" java.lang.NullPointerException
	at java.awt.Container.addImpl(Unknown Source)
	at java.awt.Container.add(Unknown Source)
	at javax.swing.JFrame.addImpl(Unknown Source)
	at java.awt.Container.add(Unknown Source)
	at ui.cfg.FrameConfig.<init>(FrameConfig.java:25)
	at ui.cfg.FrameConfig.main(FrameConfig.java:57)
找了十多分钟，终于发现
 private JTabbedPane createMainPanel()
{ return null; } 
这个方法的renturn出错了，不应该return null，而应该return new JTabbedPane()才对，改完之后运行就没问题了。